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Topic: Landing the Eagle (Read 451 times)

These were passed to me by some friends: first a chart that combines Armstrong's heart rate [in crimson], the altitude & descent rate of the L.M. [in blue], program points/alarms [in yellow], abort count [in red] and G.E.T. [in grey].

Under that, by the same author, is a 4K YouTube clip (39 mins in length).

I hadn't heard all of these loops before so I was particularly interested to hear the telecom network guys report signal acquisitions and signal strengths. When referring to a receiver, "lockup" means that it has detected, acquired and is tracking the spacecraft carrier signal with a "phase lock loop", a circuit for matching a noisy received sine wave (like a radio carrier) with a locally generated sinewave of matching frequency and phase. This is needed to further demodulate the signal, with Doppler tracking data as a side product.

You'll also hear references to "locking up on a sidelobe", which means something different. As a high gain antenna like that on the LM is pointed away from the correct direction, the signal gets weaker, reaches a minimum, and rises to another peak with a lower amplitude than when pointed in the right direction. These smaller response peaks are called sidelobes, and they can confuse the automatic tracking mechanism into tracking one instead of the desired mainlobe. The Telcom ground controller erroneously thought this was happening with the LM antenna, which was tracking the continuous uplink from earth. Later somebody figured out the real reason for the comm problems: the last-minute addition of the RCS plume detectors to the LM. They hadn't been taken into account in LM orientation planning, so the LM antenna was trying to look through one at the earth. Later during powered flight, they try an unscheduled yaw maneuver that helped greatly. LM pitch was also changing, with the earth rising higher in the sky.

Sidelobe lock is much less likely to happen today. An onboard computer can use orientation information from an IMU plus navigation data about its own position and that of the earth to generate "open loop" antenna pointing estimates to get the antenna well within its main lobe. Then the closed loop tracking servo can take over without being fooled. The LM computer wasn't powerful enough to do this, so it was up to Aldrin to manually slew (steer) the antenna where he thought it should be, then go back to automatic closed-loop tracking.

Signal strengths are given in dBm, decibels relative to 1 milliwatt. These are always negative numbers, so if somebody says "120" they really mean -120 dBm, i.e, 1e-12 times 1 milliwatt, or 1 millionth of a millionth of a milliwatt. Larger (more negative) values indicate a weaker signal. More than one size antenna was being used, and the 210' (now 70 meter) antennas naturally got stronger signals.

I never knew, or had forgotten, about the communication issues during the descent, and that CapCom had bounced a few messages to Eagle through Collins. I wonder, if the issues had continued, were they serious enough to have caused an order to abort?

I hadn't heard all of these loops before so I was particularly interested to hear the telecom network guys report signal acquisitions and signal strengths. When referring to a receiver, "lockup" means that it has detected, acquired and is tracking the spacecraft carrier signal with a "phase lock loop", a circuit for matching a noisy received sine wave (like a radio carrier) with a locally generated sinewave of matching frequency and phase. This is needed to further demodulate the signal, with Doppler tracking data as a side product.

You'll also hear references to "locking up on a sidelobe", which means something different. As a high gain antenna like that on the LM is pointed away from the correct direction, the signal gets weaker, reaches a minimum, and rises to another peak with a lower amplitude than when pointed in the right direction. These smaller response peaks are called sidelobes, and they can confuse the automatic tracking mechanism into tracking one instead of the desired mainlobe. The Telcom ground controller erroneously thought this was happening with the LM antenna, which was tracking the continuous uplink from earth. Later somebody figured out the real reason for the comm problems: the last-minute addition of the RCS plume detectors to the LM. They hadn't been taken into account in LM orientation planning, so the LM antenna was trying to look through one at the earth. Later during powered flight, they try an unscheduled yaw maneuver that helped greatly. LM pitch was also changing, with the earth rising higher in the sky.

Sidelobe lock is much less likely to happen today. An onboard computer can use orientation information from an IMU plus navigation data about its own position and that of the earth to generate "open loop" antenna pointing estimates to get the antenna well within its main lobe. Then the closed loop tracking servo can take over without being fooled. The LM computer wasn't powerful enough to do this, so it was up to Aldrin to manually slew (steer) the antenna where he thought it should be, then go back to automatic closed-loop tracking.

Signal strengths are given in dBm, decibels relative to 1 milliwatt. These are always negative numbers, so if somebody says "120" they really mean -120 dBm, i.e, 1e-12 times 1 milliwatt, or 1 millionth of a millionth of a milliwatt. Larger (more negative) values indicate a weaker signal. More than one size antenna was being used, and the 210' (now 70 meter) antennas naturally got stronger signals.

How was the antenna looking through/around the plume detectors? The antenna is at the top of the ascent stage and all the RCS and detectors are below? I'm challenged on this one.

Logged

Truth needs no defense. Nobody can take those footsteps I made on the surface of the moon away from me.Eugene Cernan

How was the antenna looking through/around the plume detectors? The antenna is at the top of the ascent stage and all the RCS and detectors are below? I'm challenged on this one.

My guess is it's the geometry of the situation.

The LM started the landing burn about 17 minutes after AOS. The burn required that the LM have its engine bell pointed forward.

Therefore, around the time of ignition, the Earth would have been located almost directly underneath the LM. I can see how a deflector might have got in the way, and how that could be alleviated by rotating the LM around its vertical axis.

How was the antenna looking through/around the plume detectors? The antenna is at the top of the ascent stage and all the RCS and detectors are below? I'm challenged on this one.

My guess is it's the geometry of the situation.

The LM started the landing burn about 17 minutes after AOS. The burn required that the LM have its engine bell pointed forward.

Therefore, around the time of ignition, the Earth would have been located almost directly underneath the LM. I can see how a deflector might have got in the way, and how that could be alleviated by rotating the LM around its vertical axis.

They were descending almost due west, facing west after pitching up all to keep the vision to be not towards the Sun. Since I don't have Celestia or similar software so I don't know where the Earth would be in relationship to the LM. If it was "below" the LM I suppose your description might be accurate until the pitch over. I'm not sure where all the communication problems existed, but I'll go with your and ka9q ideas.

Logged

Truth needs no defense. Nobody can take those footsteps I made on the surface of the moon away from me.Eugene Cernan

This is explained in detail in the Anomalies section of the Apollo 11 Mission report, near the end. The section is titled "Steerable antenna acquisition". It shows a plot graphing the antenna pointing angles against the "keep out" area blocked by the structure of the LM and the extra area blocked by the RCS deflectors.

A contributing explanation is multipath off the lunar surface. Just after earthrise, the earth is still close to the lunar horizon so some signal can bounce off the lunar surface before reaching the LM. With the direct path blocked, the autotrack may have been trying to lock onto the reflection off the lunar surface, which would be quite weak.

The mission report elsewhere gives LM pitch (relative to landing site vertical) vs time, but only during powered descent. I can't easily find whether the LM was in inertial attitude hold or in local horizontal/local vertical between AOS and the start of powered descent, so I can't tell what the antenna angles were before powered descent (the plot mentioned earlier doesn't have time marks on the angle trajectory). But if the LM was in LHLV mode so the crew could easily see the surface, then the descent engine nozzle would have been pointed forward and that would have it pointing right at the earth just after AOS.